Composition and process for treating a zinciferous surface

ABSTRACT

To inhibit the blackening of zinciferous metal-plated steel sheet while also generating a good corrosion resistance, fingerprint resistance, and chromium fixation ratio for the steel sheet, it is coated with an aqueous liquid composition that has a pH not exceeding 2.5 and contains hexavalent chromium ion, trivalent chromium ion, nickel ion, inorganic acid (preferably phosphoric acid), and film-forming resin, with a weight ratio between the nickel content and the total chromium content of 0.05 to 1, and the liquid coating is dried onto the treated metalsurface to form a blackening resistant coating that contains from 0.1 to 10 g/m 2  resin and from 2 to 150 mg/m 2  of total chromium.

TECHNICAL FIELD

The invention relates to a composition and process for treating thesurface of zinciferous metal, particularly of steel sheet coated withzinciferous metal. The invention will be generally illustrated below byuse of metal-plated steel sheet, but other zinciferous surfaces areequally suited to treatment according to the invention.

More specifically, the invention relates to a composition and process,for treating the surface of zinciferous metal-plated steel sheet, thatare capable of inhibiting both the development of white rust and thedevelopment of blackening on the surface of zinciferous metal-platedsteel sheet.

In this application, zinciferous metal-plated steel sheet is a generaldesignation for steel sheet that is plated with zinc or zinc alloy. Saidzinc alloy encompasses, for example, zinc/aluminum alloys, which mayadditionally contain one or more of iron, magnesium, manganese, silicon,titanium, nickel, cobalt, molybdenum, lead, tin, chromium, and rareearths such as La, Ce, Y, and Nb.

BACKGROUND ART

Zinciferous metal-plated steel sheet has an excellent corrosionresistance and for this reason is widely used in applications such as,for example, building materials, household electrical appliances, andautomobiles. Zinciferous metal plated steel sheet is frequently notpainted when used as a structural member or as an interior component ofhousehold electrical appliances.

Zinciferous metal-plated steel sheet is ordinarily subjected to achromate treatment in the case of such unpainted service in order toinhibit white rust. A surface-treatment process has also recentlyappeared that uses a resin containing chromate treatment bath. Thisprocess increases the added value by increasing the corrosionresistance, pressability, fingerprint resistance, and chromium fixationratio of zinciferous metal-plated steel sheet.

However, when unpainted zinciferous metal-plated steel sheet that hasbeen subjected to a conventional surface treatment as described above isused or stored in the atmosphere or in a very humid environment, itssurface develops a gray/black appearance and its commercial value isstrongly diminished as a result. This phenomenon is known as blackening.Blackening leads merely to a change in the appearance of zinciferousmetal-plated steel sheet, but leaves its other properties almostunaltered. A vexing problem posed by blackening is that it tends todevelop more readily as the corrosion resistance of the materialincreases.

Various processes for preventing blackening have already been proposed.For example, Japanese Patent Publication Number Hei 1-53353[53,353/1989] teaches a process for treating the surface of Zn/Alalloy-plated steel sheet in order to prevent blackening. This processuses a treatment bath that contains chromic acid and/or chromate salt incombination with water soluble resin or emulsified resin. However, theresults afforded by this process are still not completely satisfactory.

Japanese Patent Publication Number Hei 3-49982 [49,982/1991] alsodiscloses a process for preventing blackening. In this process, hot-dipzinciferous metal-plated steel sheet is first treated with a bath thatcontains Co ion and/or Ni ion in order to deposit these metals on thesurface. The surface of the sheet is then subjected to a chromatetreatment. This process, however, consists of a two-step method (Niand/or Co plating treatment and chromate treatment) or a three-stepmethod (the preceding plus a water wash after the Ni and/or Co platingtreatment) and is therefore difficult to implement in existing surfacetreatment facilities.

DISCLOSURE OF THE INVENTION Problems to be Solve by the Invention

In order to solve the problems described above for the heretofore knownsurface treatment agents and treatment processes for the purpose ofblackening inhibition, the present invention seeks to introduce asurface-treatment composition and a surface-treatment process that canimpart an excellent corrosion resistance, excellent pressability,excellent fingerprint resistance, and high chromium fixation ratio tozinciferous metal-plated steel sheet and that, by a simple procedure,can form thereon a surface coating layer that strongly inhibitsblackening.

SUMMARY OF THE INVENTION

The inventors have found that the problems described above can beeliminated by the addition of trivalent chromium ion and nickel ion to a(Cr⁶⁺ +resin) containing aqueous surface-treatment composition forapplication to zinciferous metal-plated steel sheet and by controllingthe nickel content therein to specific proportions. The presentinvention was achieved based on this discovery.

In specific terms, a composition according to the invention for treatingthe surface of zinciferous metal-plated steel sheet is a liquid aqueouscomposition that has a pH not exceeding 2.5 and that comprises,preferably consists essentially of, or more preferably consists of,hexavalent chromium ion, trivalent chromium ion, nickel ion, inorganicacid, and resin material composed of at least 1 type of water-basedemulsified resin, with the characteristic feature that the nickelcontent therein is adjusted so as to give a weight ratio between thenickel content and the total chromium content of 0.05 to 1.

A process of the present invention for treating the surface ofzinciferous metal-plated steel sheet characteristically comprises stepsof:

(I) application to the surface of zinciferous metal-plated steel sheetof a liquid coating of an aqueous surface treatment agent according tothe invention as otherwise described herein;

and

(II) subsequently drying the liquid film of the aqueous surfacetreatment agent applied in step (I) in order thereby to form on thetreated metal a blackening resistant dry coating layer, which preferablycontains 0.1 to 10 g/m² of the aforesaid resin material and 2 to 150mg/m² of total chromium.

DETAILED OF PREFERRED EMBODIMENTS

The inorganic acid used in the present invention preferably consists ofone or more selections from phosphoric acid, nitric acid, fluosilicicacid, fluozirconic acid, boric acid, and fluoboric acid. The nickel ionis preferably supplied by addition to the subject aqueous composition ofone or more nickel compounds selected from nickel carbonate, nickeloxide, and nickel hydroxide. The aforementioned resin materialpreferably includes at least 1 selection from water-based emulsifiedpolyacrylic resins and water-based emulsified polyurethane resins. Theinorganic acid is preferably phosphoric acid, and the phosphoric acidcontent, which is to be understood for this purpose as thestoichiometric equivalent as phosphoric acid of not only anyundissociated phosphoric acid present in the solution but also of anyphosphorus containing anions produced by any degree of ionization ofphosphoric acid, is preferably from 5 to 30 times (on a weight basis) ofthe nickel content.

The hexavalent chromium ion present in the surface-treatment compositionof the present invention can be supplied by addition to the aqueouscomposition of one or more selections from among the hexavalent chromiumcompounds heretofore used for chromate treatment baths, for example,chromic acid, chromic anhydride, chromate salts (e.g., ammoniumchromate), and dichromate salts (e.g., ammonium dichromate). Thehexavalent chromium ion acts to improve the corrosion resistanceconferred on zinciferous metal-plated steel sheet by the surface coatinglayer afforded by the present invention.

The trivalent chromium ion present in the surface-treatment compositionof the present invention can be supplied by reduction of hexavalentchromium ion and/or by addition to the aqueous composition of at least 1trivalent chromium compound, for example, chromium phosphate, chromiumnitrate, or chromium hydroxide. The trivalent chromium ion is thecomponent effective for increasing the chromium fixation ratio in thesurface coating layer afforded by the present invention on zinciferousmetal-plated steel sheet.

The nickel ion used in the present invention may be supplied by theaddition of nickel chromate or nickel dichromate to the aqueouscomposition, but is preferably supplied by the addition of at least 1nickel compound selected from nickel carbonate, nickel oxide, and nickelhydroxide. A particularly important feature of the present invention isthat the nickel ion does not impair the excellent properties ofzinciferous metal-plated steel sheet, while at the same time it isparticularly effective for inhibiting the blackening associated withresin containing chromate treatment systems. Other metal ions, forexample, even cobalt ions, do not exhibit this anti-blackening activity.

The inorganic acid used in the present invention is selected frominorganic acids that are capable of adjusting the pH of the aqueoussurface treatment composition to 2.5 or below and preferably consists ofat least 1 selection from phosphoric acid, nitric acid, fluosilicicacid, fluozirconic acid, boric acid, and fluoboric acid. Among theseinorganic acids, the use of phosphoric acid is preferred. Satisfactorilyachieving the desired anti-blackening activity becomes highlyproblematic when the pH of the surface-treatment composition exceeds2.5.

The resin material used by the present invention comprises at least 1type of water-based emulsified resin and has the capacity to form afilm. This resin material should be selected from those that are stable(i.e., exhibit no gelation or precipitation) even at pH ≦2.5 and thatresist oxidation by the chromic acid, dichromic acid, or salts thereofpresent in the surface-treatment composition. The subject resin materialshould be selected as is appropriate, as a function of the stabilitydesired for the surface-treatment composition, but in general preferablyincludes at least 1 selection from water-based emulsified polyurethaneresins and water-based emulsified polyacrylic resins, e.g., polymers orcopolymers of at least 1 selection from acrylic acid, methacrylic acid,acrylonitrile, acrylates (e.g., n-butyl acrylate, 2-ethylhexyl acrylate,etc.), methacrylates (e.g., methyl methacrylate, methoxyethylenemethacrylate, etc.), styrene, and the like. These resins exhibit anexcellent weather resistance and fingerprint resistance and are highlyadherent for a variety of paint films. The resin material may containwater-soluble resin.

The hexavalent chromium ion concentration and trivalent chromium ionconcentration in the surface-treatment composition of the presentinvention are each preferably 2 to 5 g/L. The nickel ion concentrationis preferably 1 to 3 g/L. The inorganic acid is preferably present inthe quantity necessary to adjust the pH of the aqueous composition to≦2.5, for example, 3 to 10 g/L.

The resin material is preferably present in the surface-treatmentcomposition of the present invention at a concentration of 100 to 300g/L as weight of solids.

The nickel content in the surface-treatment composition of the presentinvention is adjusted so as to give a weight ratio between the nickelcontent and the total chromium content of 0.05 to 1. The anti-blackeningactivity is inadequate when this parameter is less than 0.05, while thecorrosion resistance becomes unsatisfactory when this parameter exceeds1.

The inorganic acid content in the surface-treatment composition of thepresent invention is preferably from 5 to 30 times (on a weight basis)that of the nickel. Adjusting the inorganic acid/nickel weight ratio inthis manner affords the maximum level of anti-blackening activity.

In a process according to the invention, the aqueous surface-treatmentagent as described above is coated on the surface of zinciferousmetal-plated steel sheet and is then solidified by drying. No particularlimitations apply to the application technique, and typical applicationtechniques, for example, roll coating, roll squeegee coating, and soforth, can be used here. The technique for drying/solidification is alsonot specifically restricted, and hot-air drying, infrared drying, etc.,can be used. Hot-air temperatures of 100° C. to 400° C. are preferred,or the steel sheet can be heated to 60° C. to 250° C.

The surface-treatment composition preferably should be applied in theinvention process so as to yield a resin material content of 0.1 to 10g/m², more preferably 0.5 to 3 g/m², and a total chromium content of 2to 150 mg/m² and preferably 10 to 70 mg/m² in the blackening-resistantcoating layer formed by drying and solidification.

The fingerprint resistance becomes inadequate when the resin materialcontent falls below 0.1 g/m². On the other hand, the final product has areduced weldability and the economics become undesirable when the resinmaterial content exceeds 10 g/m². The anti-corrosion activity of thefinal coating layer is inadequate when the total chromium content fallsbelow 2 mg/m², while a total chromium content in excess of 150 mg/m²results in an unsatisfactory blackening inhibition by the final coatinglayer and also results in a reduced chromium fixation ratio.

In addition to the components described above, the surface-treatmentcomposition in accordance with the present invention may contain, forexample, cobalt ion, manganese ion, zinc ion, SiO₂, nitrate ion, and/orammonium ion.

EXAMPLES

The present invention is described in greater detail by the followingworking examples. Product performance was evaluated by the followingtests in the working and comparative examples that follow.

(a) Blackening Test

The test specimen was held for 24 hours in a humidity tester at 80° C.and 98 % relative humidity, and the change in the lightness (ΔL) beforeand after testing was measured with a color difference meter and scoredon the following scale:

    ______________________________________                                        Symbol      Change in Lightness (ΔL)                                    ______________________________________                                        ++          less than 8 (passes)                                              +           from 8 up to but not including 12 (passes)                        x           12 or more (fails)                                                ______________________________________                                    

(b) Corrosion Resistance

The test sample was subjected to salt-spray testing for 240 hours inaccordance with JIS Z 2371. It was then visually inspected and the areaof white rust development was scored on the following scale:

    ______________________________________                                        Symbol     Area of White Rust Development                                     ______________________________________                                        ++         less than 5% (passes)                                              +          from 5% up to but not including 20% (passes)                       x          20% or more (fails)                                                ______________________________________                                    

(c) Fingerprint Resistance

A finger was pressed against the test specimen and the resultingfingerprint impression was visually inspected and scored on thefollowing scale:

    ______________________________________                                        Symbol          Fingerprint Impression                                        ______________________________________                                        ++              almost not visible (passes)                                   +               slightly visible (passes)                                     x               clearly visible (fails)                                       ______________________________________                                    

(d) Cr Fixation Ratio

The Cr add-on was measured before and after subjecting the test specimento alkaline degreasing. Alkaline degreasing consisted of a 2 minutespray at 60° C. with 2 % aqueous FINECLEANER™ 4326T from NihonParkerizing Company, Limited. The proportion of Cr remaining on the testspecimen was calculated and scored on the following scale:

    ______________________________________                                        Symbol     Residual Cr Ratio                                                  ______________________________________                                        ++         85% or more (passes)                                               +          from 60% up to but not including 85% (passes)                      x          less than 60% (fails)                                              ______________________________________                                    

Examples 1 to 21 and Comparative Examples 1 to 8

Examples 1 to 21 and Comparative Examples 1 to 8 used steel sheetspecimens selected as reported in Tables 1 and 2 from steel sheets (A),(B), and (C) described below, all with plating weight=90 g/m² :

(A) hot-dip zinc-plated steel sheet

(B) steel sheet plated with 55 % Al/Zn alloy

(C) steel sheet plated with 5 % Al/Zn alloy.

The surface-treatment compositions were prepared using the componentsdescribed below.

    ______________________________________                                        Component         Quantity in g/L                                             ______________________________________                                        chromic anhydride (hexavalent                                                                   0.2 to 35 (as hexavalent chromium)                          chromium compound)                                                            reduced chromium (trivalent                                                                     0.1 to 20 (as trivalent chromium)                           chromium compound)                                                            nickel carbonate (nickel compound)                                                              0.2 to 10 (as nickel)                                       phosphoric acid   1 to 50                                                     resin             100 to 300                                                  aqueous ammonia   for pH adjustment                                           ______________________________________                                    

Tables 1 and 2 report the phosphoric acid/nickel weight ratio, pH, andtype of resin for the surface-treatment compositions in the respectiveexamples and comparative examples. The following resins were used:

Resin A=styrene/n-butyl acrylate copolymer resin in water-based emulsionform

Resin B=polyurethane resin in water-based emulsion form.

In each example and comparative example, the surface-treatmentcomposition was applied to the plated steel sheet specimen by rollcoating. A solid dry coating layer was then produced by drying andsolidification of the liquid coating layer by raising the temperature ofthe plated steel sheet specimen to 100° C. using hot air. The resinadd-on and Cr add-on were measured on the resulting product, which wasalso subjected to the tests described above. These results are reportedin Tables 1 and 2.

Benefits of the Invention

The composition and process of the present invention for treating thesurface of zinciferous metal-plated steel sheet can prevent theblackening of this type of plated steel sheet while providing anexcellent corrosion resistance, fingerprint resistance, and chromiumfixation ratio.

                  TABLE 1                                                         ______________________________________                                        Example                                                                              Test      Chromate Treatment Bath Characteristics:                     Number Substrate Ni/Cr     PO.sub.4.sup.-3 /Ni                                                                   pH    Resin                                ______________________________________                                         1     A         60/100    5       2-2.5 Resin A                               2     A         40/100    5       2-2.5 Resin A                               3     A         20/100    5       2-2.5 Resin A                               4     A         20/100    5       2-2.5 Resin A                               5     A         20/100    5       2-2.5 Resin A                               6     A         20/100    5       2-2.5 Resin A                               7     A         60/100    5       2-2.5 Resin A                               8     A         20/100    5       2-2.5 Resin A                               9     A         10/100    5       2-2.5 Resin A                              10     A         40/100    5       1-1.5 Resin A                              11     A         40/100    5       ≦1                                                                           Resin A                              12     A         20/100    10      ≦1                                                                           Resin A                              13     B         20/100    10      ≦1                                                                           Resin A                              14     C         20/100    10      ≦1                                                                           Resin A                              15     A         10/100    30      ≦1                                                                           Resin A                              16     A         20/100    10      ≦1                                                                           Resin B                              17     A         .sup. 20/100.sup.1                                                                      10      ≦1                                                                           Resin A                              18     A         .sup. 20/100.sup.2                                                                      10      ≦1                                                                           Resin A                              19     A         .sup. 20/100.sup.3                                                                      10      ≦1                                                                           Resin A                              20     A         .sup. 20/100.sup.4                                                                      10      ≦1                                                                           Resin A                              21     A         .sup. 20/100.sup.5                                                                      10      ≦1                                                                           Resin A                              ______________________________________                                        Add-On Mass of:                                                                              Resistance to:   Cr                                            Example                                                                              Resin,  Cr,     Black-       Finger-                                                                             Fixation                            Number g/m.sup.2                                                                             mg/m.sup.2                                                                            ening Corrosion                                                                            prints                                                                              Ratio                               ______________________________________                                         1     0.2      5      ++    +      +     ++                                   2     0.2     50      +     ++     +     ++                                   3     0.2     100     +     ++     +     +                                    4     1.0     50      +     ++     ++    ++                                   5     2.0     50      +     ++     ++    ++                                   6     3.0     50      +     ++     ++    ++                                   7     5.0      5      ++    +      ++    ++                                   8     5.0     50      +     ++     ++    ++                                   9     5.0     100     +     ++     ++    +                                   10     1.0     50      ++    ++     ++    ++                                  11     1.0     50      ++    ++     ++    ++                                  12     1.0     50      ++    ++     ++    ++                                  13     1.0     50      ++    ++     ++    ++                                  14     1.0     50      ++    ++     ++    ++                                  15     1.0     50      ++    ++     ++    ++                                  16     1.0     50      ++    ++     ++    ++                                  17     1.0     50      +     ++     ++    ++                                  18     1.0     50      +     ++     ++    ++                                  19     1.0     50      +     ++     ++    ++                                  20     1.0     50      +     ++     ++    ++                                  21     1.0     50      ++    ++     ++    ++                                  ______________________________________                                         .sup.1 Same composition as Example 12 except for the addition of cobalt       carbonate in a quantity equimolar to the nickel content.                      .sup.2 Same composition as Example 12 except for the addition of manganes     carbonate in a quantity equimolar to the nickel content.                      .sup.3 Same composition as Example 12 except for the addition of zinc         carbonate in a quantity equimolar to the nickel content.                      .sup.4 Same composition as Example 12 except for the addition of cobalt       nitrate in a quantity equimolar to the nickel content.                        .sup.5 Same composition as Example 12 except for the addition of silicon      dioxide in a quantity equal to ten (10) times the weight of the nickel        content.                                                                 

                                      TABLE 2                                     __________________________________________________________________________                                 Add-On Masses:                                                                        Resistance to: Cr                        Comparative                                                                         Test Chromate Treatment Bath Characteristics:                                                        Resin                                                                             Cr             Finger-                                                                           Fixation                  Ex. No.                                                                             Substrate                                                                          Ni/Cr                                                                              PO.sub.4 /Ni                                                                      pH   Resin                                                                             g/m.sup.2                                                                         mg/m.sup.2                                                                        Blackening                                                                          Corrosion                                                                          prints                                                                            Ratio                     __________________________________________________________________________    1     A    10/100                                                                             6   2.5-3.0                                                                            A   1.0 50  x     ++   ++  ++                        2     A    10/100                                                                             4   2-2.5                                                                              A   1.0 50  x     ++   ++  ++                        3     A    2/1  2   2-2.5                                                                              A   1.0 50  x     +    ++  ++                        4     A    Note 6                                                                             --  ≦1                                                                          A   1.0 50  x     ++   ++  ++                        5     B    Note 7                                                                             --  ≦2                                                                          A   1.0 50  x     ++   ++  ++                        6     C    Note 8                                                                             --  ≦2                                                                          A   1.0 50  x     ++   ++  ++                        7     A    Note 9                                                                             --  ≦1                                                                          A   1.0 50  x     ++   ++  ++                        8     A     Note 10                                                                           --  ≦1                                                                          A   1.0 50  x     ++   ++  ++                        __________________________________________________________________________     Note 6: A composition like Example 12 except without the addition of          nickel.                                                                       Note 7: A composition like Example 12 except without the nickel compound,     phosphoric acid, and trivalent chromium compound.                             Note 8: A composition like Example 12 except without the nickel compound,     phosphoric acid, and trivalent chromium compound.                             Note 9: A composition like Example 12 except that the nickel compound has     been replaced by an equimolar quantity of the corresponding cobalt            compound.                                                                     Note 10: A composition like Example 12 except that the nickel compound ha     been replaced by an equimolar quantity of the corresponding zinc compound

The invention claimed is:
 1. An aqueous liquid composition for treatingthe surface of zinciferous metal, said aqueous liquid composition havinga pH not exceeding 2.5 and consisting essentially of water and:(A) ionscontaining hexavalent chromium, (B) trivalent chromium ions, (C) nickelions, (D) phosphoric acid, and (E) a water-based emulsifiedresin,wherein there is a weight ratio between the nickel content and thetotal chromium content in the range from 0.05 to 1.0 and there is aration by weight of phosphoric acid to nickel in the range form 5 to 30.2. A composition according to claim 2, wherein resin component (E) isselected from the group consisting of water-based emulsified polyacrylicresins and water-based emulsified polyurethane resins and theconcentration of solids in component (E) in the composition is from 100to 300 g/L.
 3. A composition according to claim 2 wherein theconcentrations of each of components (A) and (B) are from 2 to 5 g/L,the nickel ion concentration is from 1 to 3 g/L, and the concentrationof phosphoric acid is from 3 to 10 g/L.
 4. A composition according toclaim 1, wherein the concentrations of each of components (A) and (B)are from 2 to 5 g/L, the nickel ion concentration is from 1 to 3 g/L,and the concentration of phosphoric acid is from 3 to 10 g/L.
 5. Aprocess of treating a zinciferous surface with an aqueous liquidcomposition according to claim 4 and subsequently drying, so as todeposit on said zinciferous surface, from the solids content of saidaqueous liquid composition, a solid dry coating layer containing from0.1 to 10 g/m² of resin solids and from 2 to 150 mg/m² of totalchromium.
 6. A process to claim 5, wherein the solid dry coating layercontains from 0.5 to 3 g/m² of resin solids and from 10 to 70 mg/m² oftotal chromium.
 7. A process of treating a zinciferous surface with anaqueous liquid composition according to claim 1 and subsequently drying,so as to deposit on said zinciferous surface, from the solids content ofsaid aqueous liquid composition, a solid dry coating layer containingfrom 0.1 to 10 g/m² of resin solids and from 2 to 150 mg/m² of totalchromium.